1. Field of the Invention
The present invention relates to an optical information recording medium, and specifically to an optical information recording medium recordable by heat mode, as well as a method for producing a writable optical information recording medium capable of recording and reproducing information using laser light having a specific wavelength, and an optical information recording medium produced by the production method.
2. Description of the Related Art
Heretofore, optical information recording media (optical discs) on which information can be written only once by irradiating laser light have been known. The optical disc is also referred to as the writable CD (so-called CD-R) and typically comprises a transparent disc substrate having successively disposed thereon a recording layer including an organic compound such as an organic dye, a light-reflective layer including metal such as Au and a protective layer (cover layer) made of a resin containing an adhesive layer for adhesion to the recording layer. The CD-R has an advantage in that it can reproduce information using commercially available CD players, and the demand for CD-Rs has increased recently along with the spread of personal computers. Information can be recorded and reproduced by irradiating laser light from the side provided with the resin layer. For example, information is recorded on the CD-R by irradiating the CD-R with laser light in a near-infrared region (usually, laser light having a wavelength of around 780 nm). Specifically, the portion of the recording layer that is irradiated absorbs light, whereby the temperature rises locally at the irradiated portion. The rise in temperature produces a physical or chemical change (for example, formation of pits) to alter the optical properties of the irradiated portion, whereby information is recorded. The information thus recorded on the CD-R is ordinarily reproduced by irradiating the CD-R with laser light having the same wavelength as that of the laser light used to record the information and detecting a difference in reflectance between the region of the recording layer whose optical properties have been changed (recorded portion) and the region whose optical properties have not been changed (unrecorded portion).
In recent years, there has been a demand for optical information recording media having higher density, and writable digital versatile discs (so-called DVD-Rs) have been proposed in response to that demand (Nikkei New Media, extra issue entitled “DVD”, 1995) which have been practically used. A DVD-R typically comprises two transparent disc substrates that each have successively disposed thereon a recording layer containing an organic dye, a light-reflective layer and a protective layer, with the discs being adhered so that the recording layers face inward or so that protective substrates having the same disc shape as these discs are disposed on outer sides of the adhered discs. Moreover, the transparent disc substrate includes a guide groove (pre-groove) used for tracking a laser irradiated onto the CD-R, with the groove having a narrower track pitch (0.74-0.8 μm) that is equal to or less than half of that in a CD-R. Information is recorded and reproduced (played back) by irradiating the DVD-R with laser light in a visible region (usually laser light having a wavelength region ranging from 630 nm to 680 nm), whereby information can be recorded at a higher density than a CD-R.
Recently, high-vision television and networks such as the Internet have rapidly become more widespread. In addition, the start of HDVT (High Definition Television) broadcasting is near at hand. As a result, larger-capacity optical media capable of recording visual information easily and inexpensively are in demand. While DVD-Rs currently play a significant role as large-capacity recording media, the demand for media having greater recording capacity and higher density continues to escalate, and development of recording media that can cope with this demand is also needed. Thus, recording media that have a larger capacity and are able to record information at a high density by irradiating light having a shorter wavelength than that of DVD-Rs are being developed. Since writable optical information recording media capable of recording information only once are being used with increasing frequency for long-term storage and back-up of large amounts of data, there is high demand for development thereof.
Methods for recording information on and reproducing information from an optical information recording medium including a recording layer containing an organic dye, by irradiating, from the side of the medium disposed with a light-reflective layer, the medium with laser light having a wavelength of 530 nm or less, are disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818 and 2000-228028. In these methods, information is recorded and reproduced from an optical disc having a recording layer containing a porphyrin compound, an azo-based dye, a metal azo-based dye, a quinophthalone-based dye, a trimethine cyanine dye, a dicyanovinylphenyl-skeleton dye, a coumarin compounds and a naphthalocyanine compound or the like, by irradiating the optical disc with a blue laser (wavelength: 430 nm, 488 nm) or a blue-green laser (wavelength: 515 nm).
On the other hand, DVDs as the phase change optical disc have been known, which adopt a system of instantaneously heating a recording layer containing an alloy layer such as GeSbTe by irradiating laser light to cause a phase change from a crystalline state to an amorphous state, whereby information is recorded and reproduced by utilizing the change in reflectance in accordance with the phase change. Usually, a recording density of optical information recording media can be increased by shortening a wavelength of laser light for recording and reproducing information and by providing a smaller beam spot by making a numerical aperture (NA) of an objective lens larger. Recently, rapid progress has been made in the development of lasers, from red semiconductor lasers having wavelengths of 680 nm, 650 nm and 635 nm to blue-violet semiconductor lasers (hereinafter referred to as blue-violet lasers) having wavelengths of 400 nm to 500 nm which are capable of recording information at ultra-high densities, and corresponding optical information recording media are also under development. Particularly, since blue-violet lasers became commercially available, an optical recording system utilizing a blue-violet laser and a high NA pick-up has been studied, and a writable optical information recording medium having a phase change recording layer and such an optical recording system has already been reported as a DVR system (“ISOM 2000” pages 210-211). This system has achieved a certain degree of progress toward increasing the recording density of the writable optical information recording media.
In the optical information recording medium used with the blue-violet laser light source, the recording layer formed on the substrate is covered with a thin cover layer having 0.1 mm-0.3 mm thickness. The system using the optical information recording medium described above condenses the beams of laser light using an objective lens having a high numerical aperture (NA) for pick-up and irradiates laser light from the side provided with the thin cover layer of the optical disc for recording information, to thereby realize an increased capacity as large as 22.5 Gbite as the one-side recording capacity.
In the optical information recording medium for use in the optical recording system that utilizes the blue-violet laser and the high NA pick-up described above, it is preferable to reduce the thickness of the cover layer for focusing the objective lens having a high NA when the blue-violet laser light is irradiated to the recording layer. As the cover layer, for example, a thin film is used and adhered to the recording layer using an adhesive or a pressure sensitive adhesive. The thickness of the cover layer, including the adhesive layer formed by curing the adhesive, is usually about 100 μm and may vary depending on the wavelength of the laser to be irradiated and the NA.
The cover layer is formed in the optical information recording medium, usually by adhering a cover film to a recording layer using a UV-curable adhesive. If the recording layer contains an organic dye, an interlayer having a predetermined thickness is disposed between the recording layer and the cover layer in order to prevent the dye from eluting by the action of the UV curable adhesive, which is a liquid. However, as the thickness of the interlayer increases, there arises a problem in that the transmittance of the laser light is reduced, resulting in lowered reflectance of the optical information recording medium.
JP-A No. 2000-67468 describes a method for forming a cover layer by continuously forming a film laminate having a constitution in which a laminate of a transparent film and a pressure sensitive adhesive sheet is provided between releasable films, then fabricating the same into a disc-shape to form a disc-shape film laminate, followed by adhering the resultant laminate to a surface of an optical disc substrate provided with a recording surface so as to match positioning thereof, while peeling off the releasable film at the side in contact with the pressure sensitive sheet.
According to the method described above, when the film laminate is continuously formed and then wound into a roll, a surface shape of the releasable film is transferred to the adhesive surface of the pressure sensitive adhesive sheet. Thus, if a surface roughness of the releasable film is large, a surface roughness of the adhesive surface also increases. Accordingly, the optical information recording medium provided with the cover layer that has been produced using the pressure sensitive adhesive sheet has troublesome incidences of surface displacement, uneven thickness or air bubble contamination, and, as a result, satisfactory recording characteristics or durability cannot be obtained.